runtime (avr): fix infinite longjmp loop if stack is aligned to 256 bytes

The setjmp code sets r24 to 0 and checks if it is still 0 when it finishes.
This usually works because tinygo_longjmp stores the lower half of the stack pointer into r24.
However, r24 will be set to 0 if the stack is aligned to 256 bytes when entering the setjmp.

Manually set r24 to 1 in tinygo_longjmp to fix this issue.

src/runtime/asm_avr.S

@@ -78,5 +78,8 @@ tinygo_longjmp:
     ld r28, X+
     ld r29, X+
 
+	; Mark that we returned from a longjmp.
+	ldi r24, 1
+
     ; Jump to the PC (stored in the Z register)
     icall